BUT - my zoom buttons don't work with the CHDK - and I need to be able to frame the shot...

Are you in "alt" mode when trying to use the zoom keys? If you are, <alt> will show at the bottom of the screen. Alt mode is only used to access CHDK controls and runs scripts. To access normal Canon controls, leave alt mode. On G15, the default alt key is the "shortcut" key, although you can change it in settings.

If the zoom keys don't work even when you aren't in alt mode, that's probably a bug in the port.